Thermosetting epoxy resin compositions are widely used as adhesives and molding materials. Among different curing agents used to cure thermosetting epoxy resin compositions are imidazole latent curing agents. These imidazole latent curing agents do not cure under normal storage conditions and are thus widely used to make thermosetting epoxy resin compositions into one-pack type curable compositions that are easy to handle and are stable during storage. One example of such imidazole latent curing agents includes an agent provided in the form of microcapsules in which particles of an imidazole compound are coated with a cured epoxy resin. The imidazole compound is capable of curing epoxy resins.
However, since the coating of this microcapsule-type imidazole latent curing agent is mechanically and thermally stable, this latent curing agent must be heated to 180° C. or above under pressurized conditions to initiate the curing reaction. This makes the imidazole latent curing agent inapplicable to recently developed epoxy resin compositions that are designed to cure at low temperatures.
For this reason, new latent curing agents have been proposed that can cure epoxy resin compositions quickly at low temperature. One is a microcapsule-type aluminum chelating agent-based latent curing agent (Patent Document 1). This latent curing agent comprises particles of an aluminum chelating agent (mother particles) and fine particles of polyvinyl alcohol (child particles) hybridized to the surface of the mother particles. The aluminum chelating agent acts with a silane coupling agent to cause an epoxy resin to polymerize cationically. Another is also a microcapsule-type aluminum chelating agent-based latent curing agent (Patent Document 2). This agent comprises particles of an aluminum chelating agent (mother particles) and fine particles of a fluorine resin (child particles) hybridized to the surface of the mother particles.
The detail of the curing process of the aluminum chelating agent-based latent curing agent is described in Patent Document 1, paragraphs 0007 through 0010.    Patent Document 1 Japanese Patent Application Laid-Open No. 2002-212537    Patent Document 2 Japanese Patent Application Laid-Open No. 2002-363255